U.S. patent number 7,758,622 [Application Number 10/552,802] was granted by the patent office on 2010-07-20 for method and apparatus for influencing magnetic particles.
This patent grant is currently assigned to Koninklijke Philips Electronics N.V.. Invention is credited to Bernhard Gleich, Juergen Weizenecker.
United States Patent |
7,758,622 |
Gleich , et al. |
July 20, 2010 |
Method and apparatus for influencing magnetic particles
Abstract
The invention relates to a method and an apparatus for
influencing magnetic particles in a region of action. With the help
of an arrangement that has means for generating magnetic fields, a
spatially inhomogeneous magnetic field is generated that has at
least one zone (301) in which the magnetization of the particles is
in a state of non-saturation, whereas it is in a state of
saturation in the remaining zone. By displacing the said zone
within the region of action, a change in magnetization is produced
that can be detected from outside and that gives information on the
spatial distribution of the magnetic particles in the region of
action. Alternatively, the displacement may also be repeated so
frequently that the region of action heats up. To improve the
accessibility of the region of action, the said region is situated
outside the arrangement having means for generating magnetic
fields.
Inventors: |
Gleich; Bernhard (Hamburg,
DE), Weizenecker; Juergen (Stutensee, DE) |
Assignee: |
Koninklijke Philips Electronics
N.V. (Eindhoven, NL)
|
Family
ID: |
33185920 |
Appl.
No.: |
10/552,802 |
Filed: |
April 15, 2004 |
PCT
Filed: |
April 15, 2004 |
PCT No.: |
PCT/IB2004/050442 |
371(c)(1),(2),(4) Date: |
October 11, 2005 |
PCT
Pub. No.: |
WO2004/091721 |
PCT
Pub. Date: |
October 28, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20060213841 A1 |
Sep 28, 2006 |
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Foreign Application Priority Data
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Apr 15, 2003 [EP] |
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03101014 |
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Current U.S.
Class: |
607/105; 607/103;
128/898 |
Current CPC
Class: |
A61N
2/02 (20130101); A61N 2/06 (20130101); A61N
1/406 (20130101); A61B 5/0515 (20130101) |
Current International
Class: |
A61F
7/00 (20060101) |
Field of
Search: |
;606/27
;607/67,103,105,113,114 ;600/12 ;128/898 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0399789 |
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Nov 1990 |
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EP |
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0117611 |
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Mar 2001 |
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WO |
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03022360 |
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Mar 2003 |
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WO |
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Primary Examiner: Gibson; Roy D
Assistant Examiner: Chen; Victoria W
Claims
The invention claimed is:
1. A method for influencing magnetic particles in a region of
action located within a subject, which method has the following
steps: introducing the magnetic particles into the region of
action, generating a magnetic field using at least one coil, the
magnetic field having a pattern in space of its magnetic field
strength such that a first sub-zone having a low magnetic field
strength and a second sub-zone having a higher magnetic field
strength are formed in the region of action, which region of action
is situated outside the space surrounding means for generating the
magnetic field, and changing the position in space of the two
sub-zones in the region of action so that the magnetization of the
particles changes locally, wherein the generating step includes
generating a gradient magnetic field with a gradient coil
arrangement that reverses its direction and has a passage through
zero in the first sub-zone of the region of action.
2. The method of claim 1, wherein the magnetic field is
positionally and temporally variable to change the position in
space of the two sub-zones in the region of action.
3. The method of claim 1, having the following further steps:
acquiring signals that depend on the magnetization in the region of
action, which magnetization is influenced by the change in the
position in space, analyzing the signals to obtain information on
the spatial distribution of the magnetic particles in the region of
action.
4. The method of claim 3, wherein the signals that are induced by
the change in the magnetization in the region of action are
received and are analyzed to obtain information on the spatial
distribution of the magnetic particles in the region of action.
5. The method of claim 1, wherein the position in space of the two
sub-zones is changed for so long, and at a frequency such, that the
region of action heats up.
6. An apparatus for influencing magnetic particles in a region of
action located within a subject, comprising: an arrangement having
means for generating a magnetic field having a pattern in space of
its magnetic field strength such that a first sub-zone having a low
magnetic field strength and a second sub-zone having a higher
magnetic field strength are formed in the region of action, which
region of action is situated outside the space surrounding the
means for generating the magnetic field, and means for changing the
position in space of the two sub-zones in the region of action so
that the magnetization of the particles changes locally, wherein
the means for generating the magnetic field comprise a gradient
coil arrangement for generating a gradient magnetic field that
reverses its direction and has a passage through zero in the first
sub-zone of the region of action.
7. The apparatus of claim 6 having at least two coils arranged
concentrically one within the other, through which coils currents
flow in opposite directions of circulation in an operating
state.
8. The apparatus of claim 6 having at least one coil and at least
one permanent magnet situated inside or outside the coil.
9. The apparatus of claim 6 having a housing enclosing the
arrangement, outside which housing the region of action is situated
in front of a side of the housing.
10. The apparatus of claim 6 having a table above which the region
of action is situated.
11. An apparatus for influencing magnetic particles in a region of
action located within a subject, comprising: an arrangement having
means for generating a magnetic field having a pattern in space of
its magnetic field strength such that a first sub-zone having a low
magnetic field strength and a second sub-zone having a higher
magnetic field strength are formed in the region of action, which
region of action is situated outside the space surrounding the
means for generating the magnetic field; means for changing the
position in space of the two sub-zones in the region of action so
that the magnetization of the particles changes locally; and means
for generating at least one temporally variable magnetic field that
is superimposed on the magnetic field, for displacing the two
sub-zones in the region of action.
12. The apparatus of claim 6 having means for acquiring signals
that depend on the magnetization in the region of action, which
magnetization is influenced by the change in the position in space,
means for analyzing the signals to obtain information on the
spatial distribution of the magnetic particles in the region of
action.
13. The apparatus of claim 12 having a coil arrangement for
receiving signals induced by the change in the magnetization in the
region of action.
Description
The invention relates to a method and an apparatus for influencing
magnetic particles in a region of action.
Magnetic particles are relatively easy to detect and can therefore
be used for examinations and investigations (particularly medical
ones). A method of this kind for determining the spatial
distribution of magnetic particles in an examination zone, together
with the use of suitable magnetic particles for such a method and
an arrangement for performing the method, are described in an as
yet unpublished patent application entitled "Verfahren zur
Ermittlung der raumlichen Verteilung magnetischer Partikel"
("Method of determining the spatial distribution of magnetic
particles") that bears the German Patent and Trademarks Office's
official number DE10151778.5. This patent application will be
referred to below as D1. To allow the spatial distribution of
magnetic particles in an examination zone (i.e. a region of action)
to be determined, a spatially inhomogeneous magnetic field is
generated that has at least one zone in which the magnetization of
the particles is in a state of non-saturation, whereas in the
remaining zone it is in a state of saturation. Changing the
position of the said zone within the examination zone produces a
change in magnetization that can be detected from the outside and
that gives information on the spatial distribution of the magnetic
particles in the examination zone.
Magnetic particles can also be used to heat their surroundings
locally, particularly in medical hyperthermia. A method and a
system of this kind for the local heating of regions of an object
by variation of the magnetization of magnetic or magnetizable
substances is described in an yet unpublished patent application
entitled "Verfahren zur lokalen Erwarmung mit magnetischen
Partikel" ("Method for local heating by means of magnetic
particles") that bears the German Patent and Trademarks Office's
official number DE10238853.9. This patent application will be
referred to below as D2. Magnetic particles are situated in the
target region (i.e. region of action) of an object. To heat the
target region locally, an inhomogeneous magnetic field is generated
having a pattern in space of its magnetic field strength such that
a first sub-zone having a low magnetic field strength (the magnetic
particles are not saturated in it) and a second sub-zone (the
magnetic particles are saturated in it) that surrounds the first
sub-zone and has a higher magnetic field strength are generated in
the target region. The position in space of the two sub-zones in
the target region is then changed for so long at a given frequency
that the particles heat up to a desired temperature due to a
frequent change in magnetization.
What are described in patent applications D1 and D2 are a method
and an apparatus in which the object for examination is at least
partly enclosed by a field-generating arrangement. The
accessibility of the examination zone or target region is impaired
in this way. This may, for example, give rise to states of anxiety
in sensitive patients during medical examinations.
It is therefore an object of the invention to improve the
accessibility of the examination zone or target region.
This object is achieved by a method for influencing magnetic
particles in a region of action, which method has the following
steps: a) generation of a magnetic field having a pattern in space
of its magnetic field strength such that a first sub-zone (301)
having a low magnetic field strength and a second sub-zone (302)
having a higher magnetic field strength are formed in the region of
action, which region of action is situated outside the space
surrounding the arrangement having means for generating the
magnetic field, b) changing the position in space of the two
sub-zones in the region of action so that the magnetization of the
particles changes locally.
In the invention, a spatially inhomogeneous magnetic field is
generated by an arrangement having means for generating magnetic
fields. The means used for generating magnetic fields may, for
example, be coils through which currents flow, or permanent
magnets. The arrangement therefore generally comprises a
three-dimensional structure that is composed of the coils and/or
permanent magnets mentioned and that is of a certain extent in
space, thus enabling a space to be defined that surrounds the said
arrangement. So that the region of action through which the
magnetic field flows is freely accessible from as many directions
as possible, this region is situated outside the space occupied by
the arrangement having means for generating magnetic fields. In
contrast to this, the arrangements having means for generating
magnetic fields that are described in, for example, D1 and D2, each
constitute a Maxwell coil arrangement. The region of action of this
arrangement is situated between the two coils of the Maxwell coil
arrangement, i.e. inside the said arrangement or inside the space
that surrounds the arrangement. Because of the improved
accessibility of the region of action in the apparatus according to
the invention, there is also a reduction in the mental stresses on
a patent in medical examinations or treatments, because, for
example, the patient does not feel hemmed in.
Magnetic particles such as are described in D1 or D2, for example,
are situated in the region of action. The magnetic field in the
first sub-zone is so weak that the magnetization of the particles
deviates to a greater or lesser degree from the external magnetic
field, which means that it is not saturated. This first sub-zone is
preferably a spatially coherent zone; it may be a punctiform zone,
but also a line or a surface. In the second sub-zone (that is, in
the remaining part of the examination zone outside the first
sub-zone), the magnetic field is strong enough to keep the
particles in a state of saturation. The magnetization is saturated
when the magnetization of practically all the particles is oriented
approximately in the direction of the external magnetic field, so
that when the strength of the magnetic field is further increased,
the increase of the magnetization in this sub-zone will be
substantially less than that in the first sub-zone in response to a
corresponding increase of the magnetic field.
One possible way of changing the position in space of the two
sub-zones is for a coil and/or permanent-magnet arrangement (or
parts thereof) intended for generating the magnetic field on the
one hand, or the object containing the magnetic particles on the
other hand, to be moved relative to one another. This is a
preferred method when very small objects are being examined with
very high gradients (microscopy). By contrast, a preferred
embodiment of the present invention does not require any mechanical
movements. The position in space of the two sub-zones can be
changed relatively quickly in this case, which provides additional
advantages for the acquisition of signals that depend on the
magnetization in the region of action.
In another embodiment of the present invention, the spatial
distribution of the magnetic particles is determined. Changing the
position of the two sub-zones within the region of action causes a
variation in the (overall) magnetization in the region of action.
If, therefore, the magnetization in the region of action or
physical parameters affected thereby are measured, information on
the spatial distribution of the magnetic particles in the region of
action can be derived from these measurements. In practice, the
particles do not have identical magnetic properties. For example, a
proportion of the particles may be saturated at a magnetic field
strength at which another proportion are in a state of
non-saturation. This however produces an (additional) non-linearity
in the magnetization characteristic that, when there is a change in
the position of the two sub-zones, leads to a change in the
magnetization in the region of action.
In another embodiment, signals that are proportional to the
temporal change in the magnetization in the region of action are
acquired. If these signals are to be as large as possible, it is
important for the position in space of the two sub-zones in the
region of action to be changed as quickly as possible. To acquire
these signals, use may be made of a coil by which a magnetic field
is generated in the region of action. Preferably however, a
separate coil will be used. The change in the position in space of
the sub-zones may take place by means of a temporally variable
magnetic field. This being the case, a signal that is likewise
cyclic is induced in a coil. However, reception of this signal
proves to be difficult in as much as the signals generated in the
region of action and the temporally variable magnetic field are
active simultaneously; a distinction cannot therefore readily be
made between the signals induced by the magnetic field and those
induced by the change in magnetization in the region of action.
In yet another embodiment, the magnetic particles situated in the
region of action are heated. If the spatial position of the first
sub-zone is changed slightly, the magnetization of those particles
that are situated in the first sub-zone, or that change from the
first to the second sub-zone or vice-versa, changes when this is
done. As a result of this change in magnetization, heat losses
arise due to very well known hysteresis effects, or effects similar
to those of hysteresis, in the particles or due to the excitation
of particle movements, and the temperature of the medium
surrounding the particles is increased in a heating-up region. If
the first sub-zone of the magnetic field is displaced through the
whole of the region of action, then the heating-up region
corresponds to the region of action. The smaller the first
sub-zone, the smaller is the size of the absolute minimum
heating-up region.
Since only a relatively small amount of heat is produced by a
once-only change in magnetization, the magnetization has to be
changed more than once. The number of such changes that is required
within a given period of time (i.e. the frequency), and the related
increase in the temperature of the medium surrounding the particles
in the heating-up region, depends on the particle concentration, on
the heat produced per change (which in turn is dependent on the
particle structure and the speed of reversal of the magnetization),
and on the heat dissipation into the regions surrounding the
examination zone.
What may be considered as magnetic particles that are suitable for
the method according to the invention are, for example, those
described in documents D1 and D2. No description will therefore be
given of the magnetic particles at this point and instead the
reader is explicitly referred to documents D1 and D2.
In another embodiment, an apparatus having a gradient coil
arrangement is provided to generate the magnetic field in the
region of action. This magnetic field is zero at a point along the
axis of the windings and increases almost linearly at opposite
polarities on the two sides of this point. Only in the particles
that are situated in the zone around the point at which the field
is zero is the magnetization not saturated. In the particles
outside this zone the magnetization is in a state of saturation. A
gradient magnetic field of this kind may be generated.
In an embodiment, it is possible not only for the region of action
to be positioned outside the arrangement having means for
generating magnetic fields but also for the region of action to be
separated in space from the entire apparatus. In this case, a wall
of a housing surrounding the apparatus is, for example, situated
between the region of action and the apparatus. The method
according to the invention may be performed as soon as the object
containing the magnetic particles is in the region of action and
close to this side of the housing. In addition, the arrangement for
performing the method according to the invention that is situated
in the apparatus is protected against external influences. If the
enclosing housing is opaque, a patient is kept from seeing into the
apparatus at the time of medical examinations, investigations or
treatments and in this way the mental stress on the patient is
further reduced.
In the embodiment, the zone generated by the gradient coil
arrangement around the zero point of the field (i.e. the first
sub-zone) is shifted within the region of action by the temporally
variable magnetic field. If this magnetic field follows a suitable
pattern over time and is suitably oriented, the zero point of the
field can pass through the whole of the region of action in this
way. When this happens, either the region of action can be heated
up or the spatial distribution of the magnetic particles can be
determined.
The change in magnetization that goes hand in hand with the
displacement of the zero point of the field can be detected, and
the spatial distribution of the magnetic particles in the
examination zone can be determined from the signal measured. The
coil used for receiving the signals generated in the region of
action may in this case be a coil that is already being used to
generate the magnetic field in the region of action. There are
however also advantages in using a separate coil for reception,
because this coil can be decoupled from the coil arrangement that
generates a temporally variable magnetic field. Also, an improved
signal-to-noise ratio can be obtained with a coil, but even more so
with a plurality of coils. In analyzing the signals received, use
is made of the fact that the magnetization characteristic of the
particles is not linear in the zone in which the magnetization
changes over from the non-saturated state to the saturated state.
This non-linearity ensures that, for example, a magnetic field that
varies sinusoidally in time at a frequency f causes a temporally
variable induction at the frequency f (fundamental wave) and at
whole-number multiples of the frequency f (harmonic waves or higher
harmonics) in the zone of non-linearity. Analysis of the harmonic
waves offers the advantage that the fundamental wave of the
magnetic field, which field is active simultaneously to shift the
field-free point, has no effect on the analysis.
For other particular embodiments, the reader is referred at this
point to documents D1 and D2.
These and other aspects of the invention are apparent from and will
be elucidated with reference to the embodiments described
hereinafter.
In the drawings:
FIG. 1 shows an apparatus for performing the method according to
the invention.
FIG. 2a and FIG. 2b show the pattern of field lines generated by
the coil arrangement contained in the apparatus.
FIG. 3 shows one of the magnetic particles that is present in the
region of action.
FIG. 4a and FIG. 4b show the magnetization characteristic of
particles of this kind.
FIG. 5 is a block circuit diagram of the apparatus shown in FIG.
1.
In FIG. 1 is shown an apparatus with which the method according to
the invention can be performed. For examination purposes, a patient
positions himself directly in front of the vertical side 2a of the
housing 2. As an alternative, the apparatus shown may also be
arranged for horizontal operation. When this is the case, the side
2a of the housing extends horizontally and the patient lies on it.
For this purpose, the side 2a of the housing may be configured as a
patient table, or a patient table is mounted in addition above the
side 2a of the housing. Before an examination, a liquid or a meal
containing magnetic particles is administered to the patient 1.
A particle of this kind is shown in FIG. 3. It comprises a
spherical substrate 100, of glass, for example, which is coated
with a soft-magnetic layer 101 that is, for example, 5 nm thick and
that is composed of, for example, an iron--nickel alloy (for
example, Permalloy). This layer may be covered, for example, by
means of a coating layer 102 that protects the particle against
acids. A particle of this kind is more exactly described in D1 and
D2. FIGS. 4a and 4b show the magnetization characteristic, that is,
the variation of the magnetization M as a function of the field
strength H, in a dispersion containing such particles. It can be
seen that the magnetization M no longer changes above a field
strength +H.sub.c and below a field strength -H.sub.c, which means
that a saturated magnetization exists. The magnetization is not
saturated between the values +H.sub.c and -H.sub.c.
FIG. 4a illustrates the effect of a sinusoidal magnetic field H(t)
if no further magnetic field is active. The magnetization
reciprocates between its saturation values at the rhythm of the
frequency of the magnetic field H(t). The resultant variation over
time of the magnetization is denoted by the reference M(t) in FIG.
4a. It can be seen that the magnetization likewise changes
cyclically, by which means a similarly cyclic signal is induced
outside the coil. As a result of the non-linearity of the
magnetization characteristic, this signal is no longer purely
sinusoidal in form but contains harmonics, i.e. higher harmonics of
the sinusoidal fundamental wave. These harmonics, which can easily
be separated off from the fundamental wave, are a measure of the
particle concentration.
FIG. 4b shows the effect of a sinusoidal magnetic field H(t) on
which a static magnetic field H.sub.1 is superimposed. Because the
magnetization is in the saturated state, it is practically
uninfluenced by the sinusoidal magnetic field H(t). The
magnetization M(t) remains constant over time at this area.
Consequently, the magnetic field H(t) does not cause a change of
the state of the magnetization and does not give rise to a
detectable signal that can be detected with a suitable coil.
To allow information to be obtained on the spatial concentration of
the magnetic particles in the object for examination (the patient
in this case), there are, in and on the housing 2 of the apparatus
shown in FIG. 1, coils and pairs of coils whose magnetic fields
flow through the region of action. The region of action is situated
in this case in front of the vertical side 2a of the housing, i.e.
outside the housing 2. A first pair of coils 3 comprises the two
windings 3a and 3b that surround one another co-axially, through
which currents flow in opposite directions of circulation and whose
common axis extends approximately perpendicularly through the
vertical side 2a of the housing. The gradient magnetic field
generated in this way is shown in FIGS. 2a and 2b by means of the
field lines 300, 300a and 300b. The field lines 300a of the
magnetic field generated by the outer winding 3a are shown as solid
lines and the field lines 300b of the magnetic field generated by
the inner winding 3b are shown as dashed lines. The magnetic fields
from the two windings are superimposed on one another to form the
magnetic field indicated by the field lines 300. This field has a
gradient in the direction of the common axis of the pair of coils 3
and at one point along this axis it reaches a value of zero. The
position of this field-free point along the common axis is selected
in such a way that it is located outside the housing 2 and inside
the region of action in which the patient is situated. Starting
from this field-free point, the strength of the magnetic field
increases in all three directions in space with increasing distance
from the point. In a zone 301 (the first sub-zone) around the
field-free point, which zone is indicated, the field strength is so
low that the magnetization of magnetic particles situated in it is
not saturated. In the remaining zone outside 301 (the second
sub-zone 302), the magnetization of the particles is in a state of
saturation.
Various parameters of the arrangement can be varied to position the
field-free point along the common axis. If the intensity of the
current flowing through the winding 3a is increased or the
intensity of that flowing through the winding 3b is reduced, the
field-free point is displaced along the common axis in the
direction of the windings 3a and 3b. If on the other hand the
intensity of the current flowing through the winding 3a is reduced
or that of the current flowing through the winding 3b is increased,
the field-free point is displaced in the opposite direction. Also,
the position, and particularly the starting position, of the
field-free point can be influenced by varying the diameter of the
windings 3a and 3b. Furthermore, the sizing of the coil arrangement
must ensure that the extent of the second sub-zone 302 in space at
least corresponds to that of the region of action so that all the
magnetic particles not situated in the sub-zone 301 are kept in a
state of saturation.
The size of the zone 301 that determines the spatial resolution of
the apparatus is dependent on the one hand on the magnitude of the
gradient of the gradient magnetic field and on the other hand on
the strength of the magnetic field required for saturation. For a
consideration of more fundamental questions the reader is referred
to documents D1 and D2.
It should be pointed out at this point that for reasons of
draftsmanship the relative sizes shown in FIGS. 2a and 2b are not
to scale. The sub-zone 301, for example, is shown as too large in
relation to the diameters of the coils formed by windings 3a and
3b, and the cross-sections of the conductors forming the windings
3a and 3b (which may incidentally also be of the same size) are
shown as too large in relation to the diameters of the
windings.
If a further magnetic field is superimposed on the gradient
magnetic field in the region of action, then the zone 301 shifts in
the direction of this further magnetic field, the size of the shift
increasing with the strength of the magnetic field. If the magnetic
field superimposed is temporally variable, then the position of
zone 301 changes with time and in space accordingly. To generate
these temporally variable magnetic fields for any desired direction
in space, three further coil arrangements are provided. A coil 4
having a winding generates a magnetic field that extends in the
direction of the axis of the coils forming pair of coils 3a, 3b,
i.e. horizontally. In principle, the effect achievable with this
pair of coils can also be achieved by superimposing currents in the
same direction on the opposed currents in the pair of coils 3a, 3b,
as a result of which the current in one pair of coils will decrease
and that in the other pair will increase. It may however be of
advantage if the temporally constant gradient magnetic field and
the temporally variable vertical magnetic field are generated by
separate pairs of coils.
To generate magnetic fields that extend in space perpendicularly to
the common axis of the coils 3a and 3b, two further pairs of coils
having windings 5a, 5b and 6a, 6b are provided, which windings 5a,
5b and 6a, 6b are situated in respective small housings on side 2a
of the housing 2. The coils forming a pair of coils are so arranged
in this case that their axes are likewise situated on a common
coil-pair axis. These two coil-pair axes extend through the region
of action, are perpendicular both to one another and to the axis of
the coils in coil arrangement 3, and intersect the latter at a
common point, preferably at the field-free point of the coil
arrangement 3.
It is however also possible for the windings 5a, 5b and 6a, 6b of
the pairs of coils 5 and 6 to be arranged inside the housing 2. For
this purpose, the four windings 5a, 5b, 6a, 6b are, for example,
arranged symmetrically about the common axis of the coil
arrangement 3, with the windings forming a pair of coils being
situated opposite one another. The windings may be positioned
inside or outside the coil arrangement 3. The axes of the coils
formed by windings 5a, 5b, 6a, 6b extend parallel or at an angle
other than 90.degree. to the common axis of the coil arrangement 3,
which means that the axes of the windings forming a given pair of
coils are then no longer situated on a common axis. This
arrangement causes a magnetic field that has a component
perpendicular to the common axis of the coil arrangement 3 to be
formed, in the region of action outside the housing 2, along an
arcuate region between the windings of a given pair of coils. The
shape of the windings 5a, 5b, 6a and 6b need not necessarily be
circular and, allow the particular arcuate magnetic field to be
optimized, may also be of other shapes.
Finally, there is also shown in FIG. 1 a further coil 7 the purpose
of which is to detect signals generated in the region of action. In
principle, any of the pairs of field-generating coils 3 to 6 could
be used for this purpose. There are however advantages in using a
separate receiving coil. A better signal-to-noise ratio is obtained
(particularly if a plurality of receiving coils are used), and the
coil can be so arranged and switched that it is decoupled from the
other coils. As an alternative, the coil 7 may also be produced in
the form of an independent component that is, for example, portable
and is held by the patient in front of his gastro-intestinal
region.
FIG. 5 is a block circuit diagram of the apparatus shown in FIG. 1.
The diagrammatically indicated pair of coils 3 (the suffixes a and
b have been omitted from all the pairs of coils in FIG. 5 for the
sake of simplicity) are supplied by a controllable current source
31 with d.c. current that can be controlled--and switched on and
off--by a control unit 10. The control unit 10 cooperates with a
workstation 12 that is provided with a monitor 13 for showing
images representing the distribution of the particles in the region
of action. Inputs can be made by a user via a keyboard or some
other inputting unit 14.
The coils in the coil arrangements 4, 5, 6 receive their currents
from current amplifiers 41, 51 and 61. The waveforms over time of
the currents I.sub.x, I.sub.y and I.sub.z to be amplified, which
currents generate the desired magnetic fields, are preset by
respective waveform generators 42, 52 and 62. The waveform
generators 42, 52, 62 are controlled by the control unit 10, which
calculates the waveform over time required for the particular
examination procedure and loads it into the waveform generators. In
the course of the examination, these signals are read out from the
waveform generators and fed to the amplifiers 41, 51, 61, which
generate the currents required for the pairs of coils 4, 5 and 6
from them.
Generally speaking, a non-linear relationship exists between the
shift of the zone 301 from its position at the center of the
gradient coil arrangement 3 and the current through the gradient
coil arrangement. Moreover, all three coils must generally generate
a magnetic field if the zone 301 is to be shifted in position along
a straight line extending off the center. Where the waveform of the
currents with time is preset, this is allowed for by the control
unit, with the help of suitable tables, for example. The zone 301
can therefore be shifted through the region of action along paths
of any desired shape.
The signals that are received by the coil 7 are fed to an amplifier
72 via a suitable filter 71. The output signals from the amplifier
72 are digitized by an analog-to-digital converter 73 and fed to an
image-processing unit 74, which reconstructs the spatial
distribution of the particles from the signals and from the
position that the zone 301 is occupying at the time during the
reception of the signals.
For an elucidation of various waveforms of the signals that occur
in the pieces of apparatus shown in FIGS. 1 and 5, for an
elucidation of the displacement of the field-free point in regions
extending in two or more dimensions, and for an elucidation of the
acquisition of the signals required for the reconstruction of the
concentration of the particles in a one-dimensional object
extending in a direction x, or even in a multi-dimensional object,
such as a consideration of the mathematical questions, for example,
the reader is referred to documents D1 and D2. Also to be found in
documents D1 and D2 is a more extensive explanation of magnetic
particles that can be used in the present case.
The advantage of the method in accordance with the invention over
magnetic resonance methods consist in that it does not require a
magnet that generates a strong, spatially homogeneous magnetic
field. The requirements imposed as regards the temporal stability
and the linearity are significantly less severe than in the
magnetic resonance method, so that the construction of such an
apparatus can be significantly simpler than that of an MR
apparatus. The requirements imposed as regards the variation in
space of the magnetic field are also less severe, so that coils
with "iron cores" (a soft-magnetic core, for example, iron) can
also be used, so that they become more effective and smaller.
The method according to the invention may also be performed in
combination with an MR examination, in which case at least some of
the coils present may be used for the reception of, or for
receiving, magnetic signals.
In line with document D2, the apparatus and method illustrated in
FIGS. 1 to 5 can also be used for the local heating of the regions
surrounding magnetic particles. The coil 7 can be dispensed with
for this purpose as no signals are detected in the local
heating-up. Because the apparatus shown in FIG. 1 has similar
components to that described in D2, the methods described in D2 may
also be applied in the present case.
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